Add Possession Presentation Layer design spec

First sub-project of the "make contact feel real" arc (candle rituals,
quantum RNG, tuning, progression, escalation to follow as separate specs).
Defines the stability-score contract shared between the backend audio
degradation and frontend text-glitch halves so they can build in parallel.

docs/superpowers/specs/2026-07-23-possession-presentation-design.md

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+# Possession Presentation Layer — design
+
+First sub-project of a larger "make contact feel real" arc (candle rituals,
+quantum RNG, tuning controls, account progression, session escalation — each
+gets its own later spec). This one: make a successful Direct Contact reply
+feel like a spirit is *fighting through static to possess the channel*,
+rather than a chat bubble appearing. Cosmetic only — the stored transcript
+and reply text are unaffected; only the live presentation (text stream +
+voice) carries the effect.
+
+## Contract
+
+A single number, **stability** (`0.05`–`0.98`, higher = cleaner possession),
+computed once per reply on the backend and shared by both halves of this
+build:
+
+```python
+# backend/app/possession.py
+def compute_stability(rarity: str, magnitude: float, rng: Callable[[], float] = random.random) -> float
+```
+
+- Base by rarity: `common=0.35, uncommon=0.5, rare=0.65, mythic=0.8` (rarer
+  spirits hold the channel more steadily).
+- `+ min(0.15, magnitude / 100)` — the anomaly magnitude that most recently
+  triggered contact; a stronger signal, a cleaner line.
+- `+ rng()` jitter scaled to `±0.1`.
+- Clamped to `[0.05, 0.98]`.
+- `rng` is injectable specifically so the later quantum-RNG sub-project can
+  swap in real entropy without touching call sites.
+
+`_handle_question` in `backend/app/ws.py` computes this once per question
+(using `state.entity.rarity_tier` and the magnitude of the most recent
+anomaly in `state.anomalies`, defaulting to 50 if none yet) and adds it to
+the existing `reply_start` frame:
+
+```json
+{"type": "reply_start", "stability": 0.62}
+```
+
+## Backend half (audio degradation)
+
+`synthesize_spirit_voice(text, voice, voice_profile, instability=0.0)` in
+`backend/app/tts/piper.py` gains an `instability` param (`0.0`–`1.0`,
+i.e. `1 - stability`) that scales up the `noise_level` and `bitcrush_bits`
+already passed to `apply_effects` — do not touch `effects.py` itself, only
+the params `synthesize_spirit_voice` feeds it. Only the `"reply"` kind
+utterance in `_speak` (the Direct Contact answer) passes a non-zero
+`instability`; greetings/fragments/ambient whispers are unaffected by this
+spec.
+
+## Frontend half (text glitch renderer)
+
+`frontend/src/lib/possession.ts` — pure function(s) that take the
+true-so-far streamed text, the stability score, and a tick/frame counter,
+and return a display string with corruption bursts and brief stutters that
+self-correct. Lower stability = more frequent/longer corruption; stability
+above ~0.85 is nearly clean (a subtle flicker only). Must be deterministic
+given its inputs (seed any internal randomness from the tick counter, not
+`Math.random()`) so it's unit-testable.
+
+Wire into `frontend/src/state/seance.tsx`: extend `replyStreaming` with a
+`stability: number` field, populated from `reply_start`'s `stability`
+(default `1.0` if absent). Wire into `frontend/src/components/Transcript.tsx`:
+render the streaming line through the glitch function instead of raw text,
+driven by a ticking interval while `streaming.active`.
+
+## Testing
+
+- Backend: unit tests for `compute_stability` (rarity ordering, magnitude
+  bonus, clamping, injected rng determinism) and for the
+  `instability`→params scaling in `synthesize_spirit_voice`. One WS
+  integration test asserting `reply_start` carries a `stability` field in
+  range.
+- Frontend: unit tests for the glitch renderer's pure functions at known
+  low/high stability values (corruption rate, self-correction, determinism
+  for a fixed tick sequence).
+
+## Explicitly out of scope here
+
+New WS frame types for interleaved glitch tokens, structural UI takeover,
+candle rituals, quantum RNG source, tuning dial, account progression,
+session escalation — all separate later specs per the arc this belongs to.